1. Field of the Invention
The present invention relates to a conductive paste suitable for forming electrodes of electronic components and conductors of thick-film circuits and to a glass used therein. More particularly, the present invention relates to a conductive paste that can be fired even in a non-oxidizing atmosphere and is suitable for forming terminal electrodes of multilayer ceramic components using a base metal such as nickel or copper for inner electrodes and to a glass used therein.
2. Description of the Prior Art
Multilayer ceramic components such as multilayer capacitors, multilayer inductors, and the like are typically fabricated in the manner as follows. Unfired (green) ceramic sheets, for example, of a dielectric or magnetic material are alternately laminated with a plurality of inner electrode paste layers to obtain a non-fired laminate. Then, the laminate is cut and fired at a high temperature to obtain a ceramic body (referred to as “ceramic body” hereinbelow). Thereafter, a conductive paste in which a conductive powder and an inorganic binder powder such as glass and the like is dispersed, if necessary, together with other additives in a vehicle, is coated by any of a variety of methods such as dipping, brush coating, screen printing, and the like on the end surfaces of the inner electrodes exposed from the ceramic body, followed by drying. High-temperature firing is then conducted to form terminal electrodes electrically connected to the inner electrodes. Then, if necessary, a plated nickel layer or a plated layer of tin or alloy thereof is formed on the terminal electrodes.
Noble metals such as palladium, silver-palladium, platinum, and the like have been used as the inner electrode materials. But in recent years, base metals such as nickel, copper, and the like came into use in order to save natural resources, to reduce cost and also to prevent the occurrence of delamination and cracking caused by oxidation and expansion of palladium. As a result, conductive pastes of base metals such as nickel, cobalt, copper and the like, which can easily form good electric connection to those inner electrode materials are also used for the formation of terminal electrodes. Because those base metal electrodes are easily oxidized during firing, the firing has been conducted at a peak temperature of about 700-900° C. in a non-oxidizing atmosphere, for example, an inert gas atmosphere or a reducing atmosphere, such as nitrogen or hydrogen-nitrogen and the like.
A non-reducible glass which is stable even in firing under a non-oxidizing atmosphere has to be used as an inorganic binder for a conductive paste to be fired in the non-oxidizing atmosphere. A PbO component contained in lead-containing glass frits, which have been widely used for conductive pastes, is easily reduced. Moreover, because lead is hazardous to the human bodies and causes environmental pollution, a glass containing no lead is required.
Further, when a terminal electrode is electroplated, adhesive strength with the ceramic body is sometimes greatly decreased by an acidic electroplating solution that modifies and dissolves glass components and breaks the glass structure. Therefore, a glass is required which has not only a high adhesive strength, but also good resistance to acids so that the glass is not vulnerable to attack from acidic plating solutions.
Another problem is that because firing is conducted under an atmosphere with a small content of oxygen, organic components such as solvents and binder resins which are used as vehicles are difficult to oxidize and decompose. If sufficient burning, decomposition, removal (referred to as “binder removal” hereinbelow) are not conducted, the vehicle decomposition products are encapsulated in the film and/or partly become carbon and remain in the film. Those carbonaceous residues cause a variety of problems, such as preventing sintering, lowering the density of the resultant fired film due to pores formed by oxidation and gasification at a high temperature and decreasing the strength of the ceramics such as barium titanate constituting the ceramic body. The selection of inorganic binder is also important in terms of resolving these problems associated with binder removal.
Accordingly, a barium-containing glass and a zinc-containing glass have been comprehensively studied as a reduction-resistant glass which has a high adhesive strength with a substrate and makes it possible to provide conductors with excellent characteristics.
For example, base metal terminal electrodes of multilayer ceramic capacitors are known which use a reduction-resistant glass such as barium borate glass, barium zinc borate glass, barium zinc borosilicate glass, and the like (see U.S. Pat. No. 3,902,102). Furthermore, it is also known to use a copper paste for terminal electrodes comprising a barium borosilicate glass (see Japanese Patent Publication No. 5-234415), to use a copper paste for terminal electrodes comprising a zinc borosilicate glass of specific composition including alkali metal components and alkaline earth metal components (see Japanese Patent Publication No. 59-184511) and to use an aluminum strontium borosilicate glass for terminal electrodes (see Japanese Patent Publication No. 9-55118).
Further, there have been proposed a copper a paste for terminal electrodes using a zinc borosilicate glass (see Examined Japanese Patent Publication No. 1-51003), and a terminal electrode paste using a zinc borosilicate glass with a superior resistance to plating solutions (see Japanese Patent Publication No. 5-342907).
However, in recent years improvements on characteristics of terminal electrodes have been strongly required. Accordingly, those conventional glasses are not always fully satisfactory for terminal electrodes. In particular, although barium-containing glass has an advantage of low softening temperature so that it can be fired at low-temperatures even if lead is not contained therein, it does not have a sufficient resistance to plating solutions and permits permeation of plating solution occurring during electroplating which reduces the adhesive strength with the ceramic body, causes cracking and fracturing of the ceramic body, induces a decrease in insulation resistance, and reduces reliability of the resultant multilayer products. Another problem was that lumps or spots of glass (referred to as “glass spots” hereinbelow) locally appeared on the electrode surface preventing the formation of a uniform plated film and inhibiting soldering.
On the other hand, a zinc-containing crystallizable glass is generally known to form a reaction layer and thereby strongly adhere to the ceramic body and has excellent strength, thermal shock resistance, resistance to plating solutions, and resistance to water. However, such a glass typically has a high softening point. A problem associated with a zinc borate glass or a zinc borosilicate glass of specific composition with a low softening point is that it is difficult to obtain a uniform glass film from these glasses because they have a narrow range of vitrification and are susceptible to phase separation. Moreover, because they are crystallizable glasses, flow characteristics and crystallization behavior in the firing process are difficult to control. Yet another problem is that the temperature range in which firing can be conducted is narrow because of dependence on process conditions, in particular, because of significant variations in characteristics related to the firing atmosphere, firing temperature, and the like.
Further, some ceramic body is also known to decrease the electrode strength. Specifically, when the ceramic body is formed from a barium titanate ceramic dielectric with F characteristic specified by JIS (Japanese Industrial Standard) C6429 and C6422, which has a high dielectric constant, the zinc-containing crystallizable glass of the terminal reacts with the ceramic body in the interface zone therebetween, forming a homogeneous reaction layer, strongly adhering to the substrate and showing practically no deep permeation into the ceramic body. However, in the case of applications to a barium titanate ceramic dielectric with B characteristic specified by JIS, i.e., a flat capacity-temperature characteristic, glass components present in the terminal electrode that were melted during firing deeply permeate into the ceramic body, degrading the strength of the ceramic body. The ceramic body so degraded may be cracked or fractured when a stress causing the electrode film to peel off is applied to the capacitor, for example, in a peel strength test of terminal electrodes. As a result, the capacitor mounted on a circuit substrate or the like has poor reliability. This is apparently due to the difference in microstructure between the ceramics; ceramics with F characteristic have a relatively homogeneous structure, whereas ceramics with B characteristic has a heterogeneous structure in which the grain boundary portions thereof have a reaction activity higher than that of crystal portions. In prior art, terminal electrodes with excellent peel strength could not be obtained on such barium titanate ceramics with B characteristic.
Thus, various types of glasses that have heretofore been developed have respective advantages, but a glass making it possible to satisfy all of the requirements has not yet been obtained.